ABSTRACT - ?Chemical Tools for the Investigation and Manipulation of Protein Glycosylation? The broad goal of this proposal is the development of chemical tools that will enable the facile and robust identification of glycoproteins, inhibition of specific types of glycosylation, and the discovery of glycosylation- mediated biological interactions. The addition of carbohydrates to proteins, or glycosylation, is one of the most common forms of posttranslational modifications and is associated with various processes, including protein stability, macromolecular interactions, and cellular signaling. Unfortunately, the currently available tools for interrogating these functions fall short, which limits the study of glycosylation to a few expert labs. We plan to tackle this unmet need in three specific aims. Aim 1 builds on our development of metabolic chemical reporters, with a focus on making these chemical tools selective for different classes of glycosylation. Aim 2 proposes to directly transform selective chemical reporters into selective small-molecule inhibitors of specific glycosyltransferases. Aim 3 leverages the advantages of chemoenzymatic modification of glycosylation to install specific photo-cross-linkers onto living cells, with a focus on identifying biological interactions that are mediated by glycans.